Standard.kt一覽

TODO

@kotlin.internal.InlineOnly
public inline fun TODO(): Nothing = throw NotImplementedError()

/**
 * Always throws [NotImplementedError] stating that operation is not implemented.
 *
 * @param reason a string explaining why the implementation is missing.
 */
@kotlin.internal.InlineOnly
public inline fun TODO(reason: String): Nothing = throw NotImplementedError("An operation is not implemented: $reason")

與java的//TODO很像，最大的區別是這個TODO會拋出異常
例：

fun main(args: Array<String>) {
    TODO()
}

Exception in thread “main” kotlin.NotImplementedError: An operation is not implemented.

或者：

fun main(args: Array<String>) {
    TODO("qfxl")
}

Exception in thread “main” kotlin.NotImplementedError: An operation is not implemented: qfxl

run

/**
 * Calls the specified function [block] and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <R> run(block: () -> R): R = block()

調用block並且返回block的結果

例：

 val qfxl = run {
        println("qfxl")
        "qfxl"
    }
    println(qfxl)

或者：

  val qfxl = "qfxl".run {
        println(this)
        this.substring(0,2)//this可以省略
    }
    println(qfxl)

輸出

qfxl
qf

在run中，用this代表當前引用對象，並且調用其方法時，this可省略。
返回值是語句塊的最後一行，若最後一行語句無返回值，則整個run語句塊也無返回值

with

/**
 * Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> with(receiver: T, block: T.() -> R): R = receiver.block()

執行接收類型爲T的block並且返回block的結果

例：

  val qfxl = with("qfxl") {
        println(this)
        substring(0, 2)
    }
    println(qfxl)

輸出

qfxl
qf

also

@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }

執行參數爲當前對象的block並且返回當前對象

例：

   val qfxl = "qfxl".also {
        println(it)
        it.substring(0, 2)
    }
    println(qfxl)//返回值依然是"qfxl"

輸出

qfxl
qfxl

let

/**
 * Calls the specified function [block] with `this` value as its argument and returns its result.
 */
@kotlin.internal.InlineOnly
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)

執行參數爲當前對象的block並且block的結果

例：

  val qfxl = "qfxl".let {
        println(it)
        it.substring(0, 2)
    }
    println(qfxl)

輸出

qfxl
qf

takeIf

/**
 * Returns `this` value if it satisfies the given [predicate] or `null`, if it doesn't.
 */
@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null

條件成立返回該對象，否則返回null

例：

  val qfxl = "qfxl".takeIf {
        it.length > 5
    }
    println(qfxl)

輸出

null

或者:

  val qfxl = "qfxl".takeIf {
        it.length > 1
    }
    println(qfxl)

輸出

qfxl

takeUnless

/**
 * Returns `this` value if it _does not_ satisfy the given [predicate] or `null`, if it does.
 */
@kotlin.internal.InlineOnly
@SinceKotlin("1.1")
public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null

*與takeIf相反，條件成立返回null，否則返回該對象*
例：

 val qfxl = "qfxl".takeUnless {
        it.length > 1
    }
    println(qfxl)

輸出

null

或者:

  val qfxl = "qfxl".takeUnless {
        it.length > 5
    }
    println(qfxl)

輸出

qfxl

repeat

/**
 * Executes the given function [action] specified number of [times].
 *
 * A zero-based index of current iteration is passed as a parameter to [action].
 */
@kotlin.internal.InlineOnly
public inline fun repeat(times: Int, action: (Int) -> Unit) {
    for (index in 0..times - 1) {
        action(index)
    }
}

源碼可知

   repeat(3, {
        println("qfxl")
    })

輸出

qfxl
qfxl
qfxl

總結

返回當前對象的操作符

apply、also

返回block結果的操作符

run、with、let